This invention relates generally to semiconductor devices and more particularly to semiconductor devices adapted to operate with high levels of microwave power.
As is known in the art, it is frequently desirable to use microwave diodes in a variety of high power applications. When used in high power applications, it is generally necessary to extract from the diode the heat generated by the diode in operating at a high power output and in maintaining an acceptable direct current to microwave frequency conversion efficiency. To accomplish this, it is frequently desirable to use a plurality of individual mesa shaped diodes to distribute the heat generated among the diodes, with the individual ones of such mesa shaped diodes having a total area equal to the one of an equivalent single mesa shaped diode. When used in such applications, the plurality of mesa shaped diodes is generally mounted to a pedestal shaped common heat sink to conduct heat away from the plurality of diodes. A technique used to mount a plurality of individual diodes adapted to operate at X-band is to individually bond each of such diodes to the common heat sink. This is a relatively difficult process for relatively small X-band diodes, however, when the diodes are designed to operate at millimeter wavelengths, they are so small that mounting them individually to the common heat sink becomes an exceedingly difficult task.
As is also known in the art, an alternate technique for mounting a plurality of diodes includes forming a thick plated heatsink on a substrate and subsequently etching the mesa shaped diodes from portions of the substrate. The substrate is diced into a plurality of sets of diodes with each one of such sets having a portion of the thick plated heat sink. The heat sink side of the set of diodes is mounted to the common, pedestal shaped heat sink. This thick plated heat sink generally provides a thermally conductive means for carrying heat away from each of the diodes in the set. Further, after dicing, each thick plated heat sink supports a set of diodes during additional processing steps, such as final packaging. However, a thick plated heat sink is undesirable when the material of the common, pedestal shaped heat sink has a higher thermal conductivity than the material of the thick plated heat sink, such as when a gold plated diamond slab is used to provide part of the common heat sink. In such a case, it is thus desirable to reduce the thickness of the plated heat sink in order to minimize the thermal resistance of the diode and to thereby increase the flow of heat from the diode to the common pedestal shaped heat sink. However, as previously mentioned, a thick plated heat sink is generally required to provide structural integrity to the wafer after the diodes have been formed into mesas, and to support individual ones of such diodes in each set for final packaging because after etching the substrate to form the mesa shaped diodes, the mesa shaped diodes are supported only by the gold plated heat sink. Therefore, if the heat sink is too thin, the structure supporting the set of mesa shaped diodes may flex, bend or crease making the set of diodes difficult to handle during the subsequent photolithographic and processing steps and may also result in damage to the diodes with a resulting lower yield of acceptable devices.